Scanned probe microscopes (SPM) have passed through many phases of development in the past two decades. A critical juncture in the development of these microscopes was the application of the tube piezoelectric scanner, which had previously been used for many years in the phonograph. This scanner was used first in atomic force microscopes (AFM) [G. Binnig and D. P. E. Smith, Rev. Sci. Instrum. 57, 1688 (1986)], which has led the way in the development of a whole variety of similar microscopes that are generally classed as SPMs.
In addition to the above, a SPM design was developed which used three tube scanners, which could scan a sample by coordinated operation [K. Besoka, Surf. Sci. 181,145 (1986)]. Furthermore, a four tube scanner was also described which allowed sample scanning by coordinated operation of the piezoelectric elements [K. Lieberman, N. Ben-Ami and A. Lewis, Rev. Sci. Instr. 67, 3567 (1996)].
Finally, a feedback mechanism was patented based on tuning forks used in watches [K. Karrai and M. Haines, U.S. Pat. No. 5,641,896] for scanning the probe or the tip that is used in SPM in close proximity to the sample. The tuning fork acts to control distance and to provide a method of distance regulation for a tip relative to the sample. This patent, however, was limited in that it required a specific direction of motion of a straight tip relative to the straight tip axis and the sample, and allowed only the use of shear force rather than normal force feedback.
The Karrai and Haines mechanism was invented for, and has been used extensively in, near-field scanning optical microscopy (NSOM). In these scanned probe microscopes an optical fiber is pulled to a small conical tip and this conical tip is coated with metal to form an aperture. Light is then passed through the optical fiber and emanates from the aperture with the dimension of the aperture.
The conical tip could be attached to one of the tines of a tuning fork for distance regulation of the tip and the sample. In this method the tuning fork resonates at a specific resonant frequency, which is generally used as a reference in watches. A straight fiber ending in a conical tip as described above is attached along one of the tines of the fork in a very specific geometry with the tip extending from the tine. The rest of the fiber is placed along the length of the tuning fork and is attached on the entire length. On this assembly a frequency is imposed. As the tip/tuning fork assembly approaches a surface with the tip oscillating at the imposed frequency there is an alteration in the tuning fork oscillation amplitude and phase and this is used to alter the position of the tip so that a tip/sample distance is maintained.
As noted above, the invention of Karrai and Hines was very limiting since it could be used only in a a specific geometry of tuning fork/straight optical fiber assembly. However, of even more importance is that it became evident, during the use of such assemblies, that the characteristics of the tuning fork could be drastically affected by attaching an optical fiber tip to one of its tines, and this was independent of other geometries that were attempted [[H. Muramatsu, N. Yamamoto, T. Umemoto, K. Homma, N. Chiba and M. Fujihara, Jpn. J. Appl. Phys. 36, 5753 (1997)] and silicon cantilevers on one of the tines of a tuning fork [W. H. J. Rensen, N. F. van Hulst, A. G. T. Ruiter and P. E. West, Appl. Phys. Lett. 75, 1640 (1999)].
One alteration, that was specifically important, was the reduction in Q factor in an uncontrolled fashion. Recently [D. N. Davydov, K. B. Shelimov, T. L. Haslett and M. Moskovits, Appl. Phys. Lett. 75, 1796 (1999)], there was an attempt to address this problem. These workers suggested that one of the reasons for this alteration in Q factor was a breaking of the symmetry of the tuning fork when such a fiber tip was attached to one of the tines [D. N. Davydov, K. B. Shelimov, T. L. Haslett and M. Moskovits, Appl. Phys. Lett. 75, 1796 (1999)]. These workers indicated that the placement of the straight optical fiber tip in a position that minimized the symmetry breaking could affect the extent of alteration of the Q factor. In addition, the mass of the fiber was given as another reason for the problems with tuning fork techniques. However, no direct control on the extent of the alteration in the Q factor was achieved. The problems raised by Davydov et al are symptomatic of the general problems of the tuning fork technique.
Tube scanning systems either with or without tuning forks have previously been employed to design either microscopes for sample scanning or to design microscopes for tip scanning, but in all the years since the introduction of the tube scanner or the tuning fork for straight tip shear force feedback, it has not been possible to design a microscope that had both tip and sample scanning.
Furthermore, a method and a device for controlled loading of a tuning fork/tip assembly has not been described in the prior art, and the geometries for tip and tuning fork that this control permits, in the present invention, could not be achieved. In addition, no method previously existed to lower, in a controlled fashion, the mass of the glue required for attaching a scanned probe tip to a tuning fork, which our invention indicates is a major problem in using such a method of feedback and in achieving usable geometries in order to form a tip/sample scanning scanned probe microscope. In addition, there is no report of a tuning fork system that has been designed without the need for gluing the tip to the tuning fork. Finally, the variety of other methods of tip feedback available in accordance with the present invention and that permit the tip/sample scanning microscope system of the invention, have not previously been described.
In addition, no prior tip scanning microscope could be placed on an upright conventional optical microscope so that the lens of the upright microscope could simultaneously view the scanned probe microscope during operation. This is important, not only to position the tip in the wider field of view but also to simultaneously collect light from a sample illuminated with a tip suitable for near-field scanning optical microscopy. It is also crucial for calibration of the optical microscope image with the near-field optical and atomic force microscopy tip.
All of the above is true not only for commercial versions of SPMs but for any reported laboratory versions.